Abstract

  
  Sediment risk assessment is typically predicated on bulk sediment, concentration-based analyses. In the case of polycyclic aromatic hydrocarbons (PAHs) in sediment, our lab and co-investigators have previously shown that bulk concentration measurements do not adequately predict what levels potential receptors are exposed. The bulk of PAH mass is found in a relatively small fraction of the sediment; namely the low-density fraction. This is also the fraction that many benthic animals may be exposed to by ingestion of this material (and potentially higher animals through biomagnification). Further, abiotic desorption rates of PAHs varied greatly by sediment fraction, with observed diffusivities ranging by 1-2 orders of magnitude between the density fractions. Given that gut passage during sediment ingestion is a relatively fast process compared to long term desorption, we investigated whether the relative PAH mass enrichment in the low density phase was accompanied by higher exposure, as measured by cumulative PAH mass release. The results show that simple measurements of bulk concentration can over or under estimate the actual exposure by orders of magnitude depending on the PAH structure and sediment composition. Given that risk assessments rely on exposure coupled with biological affect (toxicity), inaccuracies in exposure predictions may be magnified. Therefore, the results of this study have important implications for improving current methods of risk assessment for contaminated sediments.